Faults and Earthquakes Fracture A crack or break
Faults and Earthquakes
Fracture - A crack or break in the Earth’s crust.
Fractures form when STRESS, the forces per unit area acting on a material, exceeds the strength of the rocks involved.
The deformation of materials in response to stress is called STRAIN.
Elastic Rebound
Stress and Strain ultimately lead to a fracture or a system of fractures, along which movement occurs, this is called a FAULT
Types of Faults Are Classified According to the Kind of Motion That Occurs on Them
Strike-Slip Horizontal Motion Dip-Slip – Vertical Motion
Strike-Slip Fault – Left Lateral
Strike-Slip Fault – Right Lateral
Dip-Slip Faults • Normal Faults: Extension • Reverse Faults: Compression –Reverse Faults are often called Thrust Faults
Dip-Slip Fault - Normal
Normal Fault Structures
Dip-Slip Fault - Reverse
Reverse Fault Structures
Earthquakes Most Earthquakes are caused by movements along faults. Most Earthquakes occur along plate boundaries.
The location within the earth where a fault rupture actually occurs is called the FOCUS
The location on the surface directly above the focus is called the EPICENTER.
The vibrations of the ground during an earthquake are called SEISMIC WAVES.
Seismic Waves
Seismology - The study of Earthquake waves.
The sensitive instruments used to record the vibrations of earthquakes are called SEISMOGRAPHS or SEISMOMETERS.
The record produced by a seismometer is called a SEISMOGRAM. This shows the arrival times of the different seismic waves at that location.
Primary waves or P-waves squeeze and pull rocks in the same direction along which the waves are traveling.
Secondary waves, or Swaves causes rocks to move up and down at right angles in relation to the direction of the waves.
P-waves are the fastest seismic waves. Remember P comes before S in the alphabet.
Some Important Earthquakes 1755 - Lisbon, Portugal • Killed 70, 000 First Scientifically Studied Earthquake 1811 -1812 - New Madrid, Missouri • Felt over 2/3 of the U. S. • Few Casualties
Some Important Earthquakes 1964 - Alaska • Killed about 200 • Wrecked Anchorage. • Tsunamis on West Coast. 1976 - Tangshan, China • Hit an Urban Area of Ten Million People • Killed 650, 000
Major Hazards of Earthquakes • • Building Collapse Landslides Fire Tsunamis (Not Tidal Waves!)
U. S. Seismic Risk
San Francisco and New Madrid Compared
Earthquakes (M. 4) 1534 -1994
U. S. Earthquakes 1973 -2002 (28, 332 events)
Seismology and Earth's Interior
Magnitude and Intensity • How Strong Earthquake Feels to Observer Magnitude • Related to Energy Release • Determined from Seismic Records • Rough correlation between the two for shallow earthquakes
Magnitude How Much Energy is Released
Magnitude is measured using the Richter Scale
Richter scale Each successive number in the scale represents an increase in seismic-wave size, of a factor of 10.
Ex. A magnitude 7 earthquake is 10 times larger than a magnitude 6 earthquake.
Each increase in magnitude corresponds to a 32 -fold increase in energy
Ex. A magnitude 7 earthquake releases 32 times more energy than a magnitude 6 earthquake.
Intensity How Strong an Earthquake Feels to the Observer
Depends On: Distance to Quake Geology Type of Building The Observer! Varies from Place to Place
Intensity measured using the Mercalli Scale
Mercalli Scale - 1 to 12 a way of measuring earthquakes based on the amount of damage done.
Earth’s Interior as Inferred by Seismology
Discontinuity - any sudden change in the properties of a material
Earth’s Interior
CRUST- outer most layer; also known as the LITHOSPHERE
Two kind of crust: CONTINENTAL & OCEANIC
Comparing Continental to Oceanic Crust
Rocks made up of: Continental – Granite Oceanic - Basalt
Relative Thickness: Continental – Thicker Oceanic - Thinner
Relative Density: Continental – Less Dense Oceanic – More Dense
Relative Age: Continental – Older Oceanic - Younger
Moho – What is it? Boundary or interface between the CRUST and the MANTLE
The Moho was discovered by a change in seismic wave speed.
Mantle Region of dense material in the Earth’s interior between the CRUST and the OUTER CORE.
Asthenosphere Plastic-like part of the Upper Mantle
Core Broken into an Outer Core and an Inner Core
Outer Core It is a liquid… They know this because of a shadow zone in the P-waves
Shadow Zone Refraction of seismic waves creates an area on the Earth’s surface where no direct P-waves appear.
No S-waves pass through the outer core because they cannot pass through liquids.
Inner Core Solid - known because P-waves speed up
The inner core is believed to be composed of IRON and NICKEL
The overall structure of the Earth
Seismic Waves in the Earth
Inner Structure of the Earth
LOCATING EARTHQUAKES
Based on the time it takes seismic waves to reach a location, the distance of that location can be determined using a P and S wave time travel graph.
Locating Earthquakes
Doing this for two stations show you 2 possible epicenters of the earthquake.
Locating Earthquakes
Using 3 or more stations will show you the location of the Epicenter
Locating Earthquakes
Locating Earthquakes - Depth
- Slides: 78
